High power microwave device



Oct. 16, 1962 J. R. M. VAUGHAN Filed Dec. 10, 1959 INVENTORZ JAMES R. M.VAUGHAN,

| TTORNEY.

United States Patent 3,059,142 HIGH POWER MTCROWAVE DEVICE James Rodney M. Vaughan, Scotia, N.Y., assiguor to General Electric Company, a corporation of New York Filed Dec. 10, 1959, er. No. 858,665 7 Claims. (Cl. 315-39) My invention relates to electric discharge devices and pertains more particularly to high power microwave electric discharge devices of the type adapted for operating with the use of a magnetic field extending therethrough and including a waveguide output coupler incorporating a dielectric window element.

Magnetrons, for example, are high power microwave devices of the type which operate with the use of a magnetic field extending therethrough and which often include waveguide output couplers as integral parts. These Waveguide couplers usually include an energy transmitting dielectric window which serves to seal the device. The window, however, is usually vulnerable to damage such as cracking and puncturing which can result from various causes and which generally destroy the operative life of the device.

In my US. patent No. 2,967,974 issued January 10, 1961 on copending US. application S.N. 797,067Magnetron Output Coupler filed Mar. 4, 1959 and assigned to the same assignee as my present invention, 1 have disclosed and claimed a means for effectively avoiding window damage by cracking. My present invention contemplates means for avoiding window damage by puncturing, which I have found rmults from a different phenomenon than that which causes cracking. Specifically, I have recognized that puncturing of the dielectric window in the output coupler of the above-described type of device can result from a multipactor discharge which can be sometimes unintentionally established and sustained between opposing conductive surfaces in a waveguide output coupler, such, for example, as between the ramps or divergent surfaces of transformer vanes. This type of discharge can occur and be sustained when certain critical conditions as regards the spacing of the opposed surfaces, operating frequency and voltage and presence of a magnetic field of a particular value are met. When occurring in a waveguide coupler adjacent the dielectric window, the multipactor discharge can cause high-energy electron bombardment of the window which can effect the mentioned puncturing of the window. I have recognized further that all of the mentioned critical conditions are particularly easily met in, for example, waveguide output couplers of the mentioned type including opposed transformer vanes having oppositely divergent ramp surfaces and where these surfaces are close to the axis of the device and are subject to relatively strong stray magnetic fields from the main operating magnetic field extending through the device.

The primary object of my invention is to provide a new and improved electric discharge device including means for prolonging the effective operating life of the device.

Another object of my invention is to provide a new and improved electric discharge device including an improved waveguide output coupler adapted for protecting the dielectric windows therein against the damaging elfects of electron bombardment.

Another object of my invention is to provide a new and improved waveguide output coupler including a dielectric window and means adapted for suppressing any tendency toward the establishment and maintenance of a multipactor discharge therein, thereby to minimize electron bombardment of the window.

Another object of my invention is to provide a new and improved magnetron including improved means for 3,659,142 Patented Get. 16, 1962 preventing stray portions of the magnetic field employed in operation of the magnetron from deleteriously affecting the life of the device and without subtracting from the desired effect of the magnetic field in the normal operation of the device.

Another object of my invention is to provide a new and improved magnetron output coupler including new and improved means for protecting a dielectric Window therein from damage due to electron bombardment.

Still another object of my invention is to provide an improved magnetron waveguide output coupler including means for avoiding damage to a window therein by structure which can be easily and inexpensively incorporated in the device.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out in particularity in the claims annexed to and forming part of this specification.

In carrying out the objects of my invention I provide a magnetron comprising an envelope and a waveguide coupler sealed to a wall of the envelope. The coupler includes a waveguide section containing a pair of transformer vanes having opposed divergent surfaces defining a slot extending parallel to the magnetic field of the magnetron and a flaring ramp region at the end of the slot. Additionally, the coupler includes a dielectric window sealed across the waveguide section adjacent the ramp region. Supported on the coupler and surrounding and overlying the ramp region is a high magnetic permeability structure eifective for diverting magnetic flux around the ramp region. The high magnetic permeability structure can comprise a pair of C-shaped elements clamped about the coupler or can comprise an annular element fitted over and made an integral part of the coupler. Additionally, when made integral, the annular element can advantageously comprise a core of a high magnetic permeability material coated with a low radio frequency loss material.

For a better understanding of my invention reference may be made to the accompanying drawing in which:

FIGURE 1 illustrates a somewhat schematic sectional view of a magnetron incorporating an embodiment of my invention;

FIGURE 2 is a sectional view taken along the lines 2-2 in FIGURE 1;

FIGURE 3 is an enlarged fragmentary diagrammatic illustration of the manner in which my invention is effective in minimizing electron bombardment of the dielectric window; and

FIGURE 4 is a somewhat schematic fragmentary sectional view of a modified form of my invention.

Referring to FIGURE 1, there is shown a magnetron comprising a cathode I mounted centrally in a plurality of cavity resonators 2. The resonators 2 may be formed by a plurality of radial vanes 3 secured at the outer ends thereof to the inner surface of the cylindrical wall 4 of the magnetron. In actual, construction, a further body portion 5 is provided to support the resonator portions and to complete the device.

The illustrated magnetron is adapted for operating in the manner well known in the art and for such operation requires the provision of means for supplying an operating magnetic field extending axially through the device or perpendicular to the electric fields normally established in the annular region between the cathode and anode vanes. This axial magnetic field is symbolized by the point designated H in FIGURE 1.

The body portion of the device also provides support for a waveguide coupler generally designated 6 which couples to one of the resonators 2 through an aperture or slot 7 in the wall 4. The waveguide coupler 6 includes a waveguide section 8 having supported therein a pair of spaced transformer vanes 10 preferably formed of copper or similarly highly conductive low radio-frequency loss metal.

As better seen in FIGURE 2, the vanes 10 jointly define a generally H-shaped waveguide slot through the coupler. Additionally, the vanes 10 are shaped to define a pair of predeterminently spaced opposed surfaces which include oppositely divergent portions or ramps 11.

Located closely adjacent the ramps 11 or outer ends of the vanes 10 is a dielectric window 12 which is suitably sealed transversely across the outer end of the waveguide section 8. As seen in FIGURE 1, the window 12 can be sealed in a metal flange 13 which, in turn, can be suitably sealed to a shoulder in an annular member 14 bonded to the end of the wave guide section 8. The member 14 also comprises part of the H-shaped waveguide section above-mentioned and therefore is also formed preferably of a low radio-frequency loss material such as copper.

The centerbar portion of the H-shaped waveguide section defined in the coupler by the spaced vanes 10 extends parallel to the operating magnetic field H of the device and, thus, any portion of the magnetic field passing through the coupler would ordinarily extend parallel to the slot defined by the transformer vanes 11, as indicated by the arrow designated H in FIGURE 2.

I have found that during normal operation of a device of the type illustrated and when a magnetic field H r of a particular value, namely the cyclotron value, is present in the transformer slot, and especially in the ramp region or space between the divergent surfaces 11, all of the conditions are met to establish a multipactor discharge between the opposed surfaces of the transformer vanes 10. That is, the operating frequency of the device and normally high operating voltage are usually appropriate during normal operation for sustaining a multipactor discharge. Additionally, because the ramp surfaces 11 are divergent there is a high degree of probability that the distance required between opposed surfaces will at some points on the ramps be the correct value to meet the phase condition requirement for a multipactor discharge. Further, stray portions of the operating magnetic field are usually strong enough in the ramp region to meet the mentioned cyclotron value required to sustain a multipactor discharge.

According to classic multipactor discharge theory, only a single electron is required to initiate a discharge, and the probability of the presence of at least one electron in the ramp region of the waveguide coupler is very high. Also according to classic theory the normal radio frequency operating voltage between the ram surfaces which is approximately 50,000 volts in a device of the type disclosed will be generally too high to sustain a multipactor discharge. However, as illustrated schematically in FIGURE 3, the mentioned stray portion of the magnetic field H extends parallel to the ramp surfaces 11 in the coupler, and I have found that this portion of the magnetic field is effective for causing an electron emitted, for example, at the point 15 to travel a tightly curved path of'the type illustrated in arrowed dash lines and thus be caused to impinge at an oblique angle upon a point 16 on the opposite ramp. It is well known to those skilled in the art that emission of secondary electrons occurs more readily for oblique impingement than for perpendicular impingement. Thus, this oblique impingement readily knocks or dislodges secondary electrons from the point 16 which is caused to travel back toward the first ramp also at an oblique angle. Many of these secondary electrons skirt past the surface of the firstmentioned ramp and continue on to strike the dielectric window in the manner indicated in FIGURE 3, thus ressulting in electron bombardment of the window and destructive puncturing thereof.

I have solved the problem of window punctures in the described type of structure by altering the magnetic field in the ramp region so as to insure that it will be below the critical value or, in other words the cyclotron value, which is critical to the sustainance of a high-voltage multipactor discharge while at the same time avoiding any undue charges of the magnetron-operating portion of the magnetic field. That is, by reducing the strength of the field H in the transformer slot between the ramps 11 I have been able to cause any electrons emitted from one ramp, such as from the point 15 in FIGURE 3, to strike the opposite ramp at a more perpendicular angle, as in the manner illustrated by the arrowed solid line in FIGURE 3. This reduces substantially the effectiveness of such electrons in causing secondary emission from the other ramp and has been found effective for minimizing electron bombardment of the window. This reduction of electron bombardment has been to the point where window life, and therefore life of the device incorporating the window, is prolonged substantially, and, in fact, to a point where window life surpasses the normal operating life of the remainder of the device.

In order to alter the strength of the magnetic field in the ramp region I have provided a magnetic shielding structure surrounding the waveguide coupler and overlying the ramp region of the transformer. The shielding structure diverts any stray magnetic flux from the operating magnetic field around the ramp region of the coupler and, thus, greatly reduces the magnetic field H between the ramps and is'efiective for avoiding the type of multipactor discharge illustrated in dash lines in FIG- URE 3 which, as pointed out above, effects electron bombardment of the window. With the field H reduced in the ramp region any electrons emanating from a ramp will, under the influence of the weakened H field, be caused to impinge upon the opposite ramp more perpendicularly, with resultant less chance of dislodging an electron which will travel back and bombard the window.

In the embodiment illustrated in FIGURE 1 and 2 the magnetic shield structure comprises a pair of C-shaped steel elements 17 which can be clamped on the coupler in a position overlying the ramp region in any suitable manner (not shown). Preferably any spaces between the ends of the C-shaped elements are aligned to be parallel with the field H, in the manner shown in FIGURE 2. In this manner, magnetic flux does not pass across the spaces and, thus, the presence of the spaces does not detract from the shielding effect of the arrangement. While the C-sh-aped elements can advantageously be made of steel any high magnetic permeability material would be suitable.

Illustrated in FIGURE 4 is a modified form of my invention wherein the shielding structure comprises an integral part of the waveguide defining structure. In this embodiment the window flange comprises an annular member 18 including a core or base 19 of high magnetic permeability material, such as steel, for effecting the above-discussed divergence or shunting of the magnetic flux around the ramp regions, thus to suppress any multipactor discharge. Additionally, inasmuch as steel has an unduly high rate of radio-frequency loss and thus is unsuitable for defining the inner surface of a waveguide, the base 19 can have a coating or plating 20 of copper, silver or any other low radio-frequency loss material. Thus, the coating 20, can serve effectively as an extension of the inner wall of the waveguide. For this purpose, only the inner surface of the base need be coated. However, in practice it is more convenient to plate the whole ring and thereby provide the desired low radio-frequency loss internal surface as well as a corrosion-resistant external surface.

The relative thickness of the C-shaped elements 17 or the base portion 19 of the member 18 is determined by the requirement that the resulting screening or shielding must reduce the magnetic field in the ramp region, or region where the multipactor discharge is to be supa pressed, to a value much less than the cyclotron value defined as Weber per square meter, where w is the angular frequency of oscillation of the tube and e and m are the charged and mass of an electron, respectively. In some magnetron applications elements thick and sufiiciently wide to extend over substantially the whole of the ramp region have been found satisfactory. In other applications a diiferent thickness may be required.

Thus, I have provided improved means for avoiding window damage due to electron bombardment and thereby have provided an improved device adapted for longer operating life. Additionally, I have accomplished these desiderata without adversely affecting the portion of the magnetic field utilized in the normal operation of the device and without resort to expensive or complicated means.

It will be understood that while I have shown and described my invention embodied in a magnetron device its application is not limited to such a device but is equally applicable to other high-power microwave devices, whether of the cross-field or magnetically focused types, such, for example, as klystrons and traveling wave tubes, and wherein the window of a waveguide structure is subject to damage from a multipactor discharge tending to be sustained under the influence of stray portions of the magnetic field.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device comprising an envelope,

means providing an operating magnetic field extending coaxially through said envelope, a waveguide output coupler including a waveguide section extending laterally outwardly of said envelope and including a dielectric window in the outer end, said coupler including a pair of impedance transformer vanes supported in spaced relation in said waveguide section and defining a slot extending parallel to said magnetic field, said vanes having opposed divergent surfaces defining a ramp region located adjacent said window, said ramp region being subject during normal operation of said device to the establishment in said ramp region of a multipactor discharge when stray portions of said magnetic field attain a critical value in said ramp region, whereby said window is subject to destructive electron bombardment, and a pair of C-shaped high magnetic permeability elements embracing said coupler in overlying relation with respect to said ramp region and with the spaces between the C-shaped members being parallelly aligned with said slot and said magnetic field, said elements diverting said stray portions of said magnetic field around said ramp region and maintaining any magnetic field in said ramp region below said critical value, thereby to suppress the establishment of a multipactor discharge in said ramp region.

2. An electric discharge device adapted for operating with a magnetic field extending therethrough comprising an envelope, a waveguide output coupler including a waveguide section extending outwardly of said envelope and including a dielectric Window in the outer end thereof, said coupler including a pair of impedance transformer vanes supported in spaced relation in said waveguide section and defining a slot extending parallel to said magnetic field, said vanes having opposed divergent surfaces defining a ramp region located adjacent to said window, said ramp region being subject during normal operation of said device to the establishment in said ramp region of a multipactor discharge under the influence of stray portions of said magnetic field, wheerby said window is subjected to destructive electron bombardment, and an annular member comprising a wall portion of said waveguide section and extending in overlying relationship with said ramp region for providing a fiux path diverting said stray 6 portions of said magnetic field around said ramp region thereby to suppress the establishment of said multipactor discharge, said annular member being of a high magnetic permeability material and having a low radio-frequency loss coating at least on the inner surface thereof.

3. An electric discharge device comprising an envelope, means providing an operating magnetic field extending coaxially through said envelope, a waveguide output coupler extending laterally outwardly of said envelope, and means surrounding said coupler and maintaining any magnetic field in said coupler at a value substantially less than the cylotron value defined as will Weber per square meter where w is the angular frequency for oscillation of the device and e and m are the charge and mass of an electron, respectively, thereby to suppress any tendency toward the establishment of a multipactor discharge in said coupler.

4. An electric discharge device comprising an envelope, means providing an operating magnetic field extending coaxially through said envelope, a Wave guide output coupler including a Wave guide section extending outwardly of said envelope and having a dielectric Window in the outer end thereof, said coupler including a pair of impedance transformer vanes supported in spaced relation in said wave guide section and having opposed divergent surfaces defining a ramp region located adjacent said window, said ramp region tending to be subject during normal operation of said device to the undesirable establishment therein of a multipactor discharge when stray portions of said magnetic field attain a critical value in said ramp region, and a high magnetic permeability element surrounding and overlying said ramp region of said coupler, said element being separate from and independent of said means providing said operating magnetic field and having a predetermined thickness effective for miantaining any magnetic field in said ramp region below said critical value, thereby to suppress said tendency toward the establishment of said multipactor discharge.

5. An electric discharge device comprising an envelope, means providing an operating magnetic field extending coaxially through said envelope, a waveguide output coupler extending laterally outwardly of said envelope and said magnetic field, said coupler including opposed divergent surfaces tending to be subject to the undersirable establishment therebetween of a multipactor discharge when stray portions of said magnetic field attain a critical value in the region between said divergent surfaces, and means separate from and independent of said means providing an operating magnetic field effective for diverting any stray portions of said magnetic field around said coupler and maintaining any magnetic field in said region between said divergent surfaces below said critical value, whereby said tendency toward establishment of said multipactor discharge is suppressed.

6. An electric discharge device comprising an envelope, means providing an operating magnetic field extending coaxially through said envelope, a Waveguide output coupler extending laterally outwardly of said envelope and said magnetic field, said coupler including opposed divergent surfaces tending to be subject to the undesirable establishment therebetween of a multipactor discharge when stray portions of said magnetic field attain a critical value in said coupler in the region between said divergent surfaces, a dielectric Window in said coupler subject to damage from electron bombardment resulting from said multipactor discharge, and a high magnetic permeability element separate from and independent of the magnetic circuit of said means providing said operating magnetic field and extending about said coupler in a plane parallel to said operating magnetic field and in the region of said divergent surfaces, said element being effective for diverting any strap portions of said magnetic field around said coupler and maintaining any magnetic field in said region of said divergent surfaces below said critical value, whereby said tendency toward establishment of a multipactor discharge is suppressed.

.7. An electric discharge device comprising an envelope, means providing an operating magnetic field extending coaxially through said envelope, a waveguide output coupler including a section extending outwardly of said envelope and said magnetic field, a dielectric window in the outer end of said waveguide section, said coupler including a pair of impedance transformer vanes supported in spaced relation in said waveguide section and defining a slot extending parallel to said magnetic field, said vanes having opposed divergent surfaces defining a ramp region located adjacent said window and remote from said op erating magnetic field, said ramp region tending to be subject during operation of said device to the undesirable establishmenttherein of a multipactor discharge when stray portions of said magnetic field attain a critical value in said ramp region, whereby said window is subjected to electron bombardment, and means separate from and independent of said means providing an operating magnetic field defining a magnetic shield surrounding said ramp region and maintaining any magnetic field in said region below said critical value, whereby said tendency toward establishment of said multipactor discharge is suppressed and electron bombardment of said Window is avoided.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,212 Bondley July 16, 1946 2,842,713 Derby July 8, 1958 2,905,847 Klien et a1 Sept. 22, 1959 2,918,593 Rogers Dec. 22, 1959 3,005,126 Cutler Oct. 17, 1961 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,059,142 October 16 1962 James Rodney M. Vaughan It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4 line 5, for "charges" read chan es 1111 37, for "FIGURE" read FIGURES column g, line 7 for charged" read charge line 71, for "wheerby" v read whereby column 6 line 39, for :"miantaining" read maintaining Signed and sealed this 30th day of April 1963,

(SEAL) Attest:

ERNEST w. SWIDER DAVID AD Attesting Officer Commissioner of Patents 

